A fuel cell, for example, a polymer electrolyte fuel cell converts chemical energy of a substance directly into electric energy through an electrochemical reaction caused by supply of reaction gases (a fuel gas and an oxidant gas) through gas diffusion layers to a membrane-electrode assembly that includes a polymer electrolyte membrane sandwiched between a pair of catalyst layers.
A gas diffusion layer of a fuel cell is produced by subjecting a porous carbon electrode substrate such as carbon paper to a water repellent treatment, and has a microporous layer on a surface in contact with a catalyst layer. When monofilaments of carbon fibers of the porous carbon electrode substrate penetrate the polymer electrolyte membrane, the performance of the fuel cell may deteriorate due to a short circuit or degradation of the polymer electrolyte membrane starting from the penetrated portion. Even though the performance deterioration initially has a small influence on the power generation, repeated swelling and shrinkage of the membrane due to repetition of starting and stopping of the fuel cell advance the short circuit and degradation of the polymer electrolyte membrane to lower the durability of the fuel cell.
Therefore, Japanese Patent Laid-open Publication No. 2010-70433 discloses a method of manufacturing a porous carbon electrode substrate from which short carbon fibers not bonded with carbonized resin have been sufficiently removed through a treatment of blowing gas onto at least one surface of the porous carbon electrode substrate.
In addition, Japanese Patent Laid-open Publication No. 2012-33458 discloses a method including disposing an insulating member having a plurality of communicating holes on a water-repellent layer side of a gas diffusion layer formed by laminating a layer made of carbon fibers and a water-repellent layer, sandwiching the gas diffusion layer and the insulating member between a pair of electrodes, disposing a pair of pressure plates on back surfaces of the pair of electrodes to sandwich the electrodes, and pressurizing the gas diffusion layer with the pair of surface pressure plates. When a voltage is applied to the pair of electrodes while the gas diffusion layer is pressurized, a current is caused to flow through the protruding portion of the carbon fibers contacting the electrode on the water-repellent layer side through the communicating holes in the insulating member, and the protruding carbon fibers are burned off with Joule heat.
Meanwhile, Japanese Patent Laid-open Publication No. 2012-204142 discloses a method including disposing an elastic sheet on at least one surface of a carbon sheet including short carbon fibers bonded with carbon, pressurizing the carbon sheet at a linear pressure of 5 kN/m to 30 kN/m using a continuous pressurizing means, and then continuously removing the carbon powder adhered to the carbon sheet by a method of sweeping the carbon powder with a brush or the like, a method of sucking the carbon powder, ultrasonic cleaning or the like.
The method described in Japanese Patent Laid-open Publication No. 2010-70433, however, has the following problem: although the surface of the gas diffusion layer can be cleaned to some extent, carbon fibers newly protrude when the gas diffusion layer is compressed, for example, in a step of joining the gas diffusion layer to a polymer electrolyte membrane and, thus, the protruding carbon fibers stick into the polymer electrolyte membrane to generate a large short circuit current.
The method described in Japanese Patent Laid-open Publication No. 2012-33458 has the following problem: the carbon fibers tend to burn off in a narrow portion of protruding carbon fibers that generates a large heat, and in the middle part in the longitudinal direction of protruding carbon fibers with little heat radiation so that the carbon fibers protruding from the gas diffusion layer to the cut portion may remain, or portions of the carbon fibers protruding beyond the cut portion may be contaminated and cause a short circuit of the polymer electrolyte membrane.
The method described in Japanese Patent Laid-open Publication No. 2012-204142 also has a problem that, since an elastic sheet is disposed on at least one surface of the carbon sheet, the pressurizing force is distributed to fold the short carbon fibers, and the effect of removing the short carbon fibers is reduced.
It could therefore be helpful to provide a porous carbon electrode substrate that hardly causes a short circuit when used in a fuel cell, and from which short carbon fibers insufficiently bonded at the substrate surface have been sufficiently removed.